Physics Lesson 10 Newton’s Third Law of Motion – Action & Reaction Eleanor Roosevelt High School Chin-Sung Lin
Physics Lesson 10 Newton’s Third Law of Motion – Action & Reaction Eleanor Roosevelt High School Chin-Sung Lin.
Dec 23, 2015
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Physics Lesson 10
Newton’s Third Law of Motion – Action & Reaction
Eleanor Roosevelt High SchoolChin-Sung Lin
Review – Newton’s First Law
Law of inertia
Every object continues in a state of rest, or of motion in a straight line at constant speed, unless it is compelled to change that state by an unbalanced force exerted upon it
No Unbalanced Force
No unbalanced force means the net force = 0
Fnet = 0
Stay at Rest orConstant Velocity
Review - Newton’s Second Law
Law of Force & Acceleration
The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object
Unbalanced Force
Unbalanced force means the net force ≠ 0
F
Fnet ≠ 0
Fnet = m a
am
Newton’s Third Law Example
Newton’s Third Law Example
Newton’s Third Law Example
Newton’s Third Law Example
Newton’s Third Law Example
Newton’s Third Law Example
Newton’s Third Law Example
Newton’s Third Law Example
Newton’s Third Law Example
Newton’s Third Law Example
Newton’s Third Law Example
Newton’s Third Law Example
Newton’s Third Law Example
Newton’s Third Law Example
Newton’s Third Law Example
Newton’s Third Law Example
Newton’s Third Law Example
Newton’s Third Law Example
Newton’s Third Law Example
Newton’s Third Law Example
Newton’s Third Law Example
Newton’s Third Law
Law of Action & Reaction (or Interaction)
Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object
To every action there is always an equal opposing reaction
Newton’s Third Law
Action force = Reaction force
Faction = Freaction
Newton’s Third Law
Action and reaction forces are acting on different objects
It doesn’t matter which one is called action and which one is called reaction
Force is not something an object has, like mass. Force is an interaction between two objects
There cannot be action force without a reaction force
System Concept
A systems is a coherent entity which has certain function
A system can be characterized and described by its input and output, and the relation between them
A system can be viewed as a collection of interconnected subsystems
Example: Identify the Interaction
Example: Identify the Interaction
Earth attracts the vase Fg
Example: Identify the Interaction
Earth attracts the vase
The vase attracts Earth
Fg
Example: Identify the Interaction
The table supports the vase FN
Example: Identify the Interaction
The table supports the vase FN
The vase pushes the table
Example: Identify the Interaction
Fg
FN
Example: Identify the Interaction
Fg
FN
If the vase is our focus (or system)
Example: Identify the Interaction
Fg = FN Fg
FN
If the vase is our focus (or system)
Exercise: Identify the Interaction
Exercise: Identify the Interaction
Fg
Exercise: Identify the Interaction
FT
Exercise: Identify the Interaction
Fg
FT
Exercise: Identify the Interaction
Fg
FT
Exercise: Identify the Interaction
Fg = FT
Fg
FT
Newton’s Third Law
Action force = Reaction force
Faction = Freaction
Do they cancel each other out?
Why Does the Man-Box Move?
A Man-Box System
Why Does the Man-Box Move?
A Man-Box SystemHorizontal forces
only
Why Does the Man-Box Move?
A Man SystemHorizontal forces
only
Why Does the Man-Box Move?
A Box SystemHorizontal forces
only
Why Does the Man-Box Move?
A Man-Box SystemHorizontal forces
only
Newton’s Third Law
Action force = Reaction force
Faction = Freaction
They do not cancel each other out since they are acting on the different objects
Why Does the Horse-Cart Move?
A Horse-Cart System
Why Does the Horse-Cart Move?
A Horse-Cart SystemHorizontal forces
only
Why Does the Horse Move?
A Horse SystemHorizontal forces
only
Why Does the Horse Move?
A Horse SystemHorizontal forces
only
Why Does the Cart Move?
A Cart SystemHorizontal forces
only
Why Does the Cart Move?
A Cart SystemHorizontal forces
only
Why Does the Horse-Cart Move?
A Horse-Cart SystemHorizontal forces
only
Why Does the Horse-Cart Move?
A Horse-Cart SystemHorizontal forces
only
Exercise: Identify the Interaction
A Rear Wheel Drive Car
Exercise: Identify the Interaction
A Front Wheel Drive Car
Exercise: Identify the Interaction
Exercise: Identify the Interaction
A 100 N pushing force at 30o angle is exerted on a 10 kg block against the wall. Identify and calculate all the forces acting on the block by drawing the free-body diagram
30o
100 N
10 kg
Exercise: Identify the Interaction
30o
100 N
10 kg
Newton’s Third Law
Action force = Reaction force
Faction = Freaction
m1 a1 = m2 a2
m1 a1 = m2 a2
Interaction on Different Masses
Interaction on Different Masses
Interaction on Different Masses
50 kg
40 kg2
m/s2a = ?
Interaction through a String
The mass of the object is 5 kg
The reading of the scale is 50 N
reading of the scale?
reading of the scale?
Interaction through a String
reading of the scale?
reading of the scale?
Interaction through a String
MA = MB = 5 kg
MC = ?
FTC = ?
FTA = ?
Interaction through a String
m
10 kg 20
kg
Interaction through a String
M2 = 5 kgΘ = 30o
T
Interaction with acceleration
At rest orconstant speed
a = 0 m/s2
Interaction with acceleration
FgAt rest orconstant speed
a = 0 m/s2
Interaction with acceleration
Fg
FN
At rest orconstant speed
FN = Fg
Scale Reading
a = 0 m/s2
Interaction with acceleration
Accelerate upward
a
Interaction with acceleration
FgAccelerate upward
a
Interaction with acceleration
Fg
FN
Accelerate upward
a
FN > Fg
Scale Reading
Interaction with acceleration
Accelerate downward
a
Interaction with acceleration
FgAccelerate downward
a
FN FN < Fg
Scale Reading
Interaction with acceleration
Accelerate Downward with a = g
a = g
Interaction with acceleration
FgAccelerate Downward with a = g
a = g
Interaction with acceleration
FgAccelerate Downward with a = g
a = g
FN = 0 N
Scale Reading = 0 N
Interaction with acceleration
Interaction with acceleration
Interaction with acceleration
Reading?
Interaction with acceleration
Frictionless
T = ?
a = 2 m/s2 m1 = 10 kg
m2 = ?
Summary
Newton’s third law - Faction = Freaction
Every action always pairs with an equal opposing reaction
Action and reaction forces won’t cancel out each other since they are acting on different objects
Identify the interactions
Use system concept and free-body diagram to identify the forces exerting on a system
Summary
Use system concept and free-body diagram to explain motion
Interaction on different masses m1 a1 = m2 a2
Interaction through a string - Tension
Interaction with acceleration
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